1. Field of the Invention
The present invention relates to a three axis thruster modulation logic for control of a spacecraft.
2. Description of Related Art
Previous methods for modulating thrusters have been described in the literature and generally referred to as "derived rate controller", "pseudo rate modulator", or "pulse width/pulse frequency modulator". There are a variety of implementations but they always contain an off/on non-linearity with deadband and hysteresis. Bittner, in U.S. Pat. No. 4,567,564, refers to this as a "relay" function. Typically, there is a feedback loop, with a first order filter, wrapped around the non-linear function. The filtered signal, being fedback, serves as a derived estimate of the spacecraft rate as it responds to the thruster firing. This signal is used to shut the firing down when the rate estimate is sufficiently large. Pointing control amplitude is controlled by adjusting the width of the deadband. Pulse frequency, and hence excitation of flexible spacecraft modes, is indirectly controlled by adjusting the hysteresis value and the time constant of the feedback path. These methods are well known to those practicing three axis spacecraft control. They are all based on analog logic to modulate a set of jets for control about a single axis. Digital implementation is alluded to, but constitutes a digitalized version of the analog logic rather than an inherently digital design.
Another disadvantage of the pseudo rate modulator is that its low amplitude response is very non-linear. At some level its output is zero, for low amplitudes it simply does not fire. This characteristic has not been a significant problem when using low level thrusters, such as thrusters with a minimum impulse of approximately 0.004 lb-sec., but becomes more of a problem when using 5 lb thrusters, with a minimum impulse of approximately 0.09 lb-sec. By contrast, the approach of this invention produces a proportional thrust impulse at any non zero level of input although it does introduce some lag time as the level becomes very low. The disadvantage associated with the lag can be overcome with a fractional modulation scheme, described later.
Bittner improves on the pseudo rate modulator in a number of ways. Bittner eliminates the filtering in the feedback path around the non-linearity by feeding the derived rate information directly back to the state estimator where it alters the estimates of rate and position. Bittner also describes additional complexity in which the deadband and hysteresis are adjusted adaptively, based on observed pointing error, amplitude of low frequency oscillations and/or derived disturbance torque levels. This complexity can be used to adjust frequency of pulsing and thereby avoid excitation of certain spacecraft structural resonances. Bittner's improvements are still confined to a single axis and are based on analog logic although Bittner discusses how to digitally mimic this logic.
Garg, in U.S. Pat. No. 4,848,706, extends use of the same type of modulator to a spacecraft where the torques produced by thrusters are not restricted to be orthogonal but the thrusters must still be arranged in groups with some thrusters of a group producing a torque opposite of that produced by the other thrusters in the group. He divides the thrusters into three groups with torque vectors not necessarily being orthogonal, then maps the control torque commands into components along the three thruster torque axes and finally applies the same pulse width/pulse frequency modulator to each thruster group. With this approach, Garg is able to achieve a minimum set of twelve thrusters per spacecraft as compared to a minimum of ten thrusters achieved by the present invention.